Recently, in order to achieve a weight saving of various members for improving fuel consumption of an automobile, thinning by high-strengthening of a steel sheet such as an iron alloy or application of a light metal such as Al alloy has been developed. Compared to a heavy metal such as steel, the light metal such as Al alloy has an advantage such as having high specific strength, but there is a disadvantage such as having significantly high costs. Thereby, the application of the light metal is limited to a specific use. Accordingly, in order to promote weight reduction of various members at lower cost and in a wider range, the thinning by high-strengthening of the steel sheet is needed.
In general, due to the high-strengthening of the steel sheet, deterioration of material characteristics such as formability (workability) is accompanied. Thereby, improvement of the high-strengthening without deterioration of the material characteristics is important in the development of a high-strength steel sheet. Particularly, a steel sheet, which is used as a vehicle member such as an inner sheet member, a structural member, a suspension member, or a transmission, requires bendability, stretch-flange workability, burring workability, ductility, fatigue durability, impact resistance (toughness), corrosion resistance, or the like according to the use. Accordingly, having an improved balance of material characteristics at a high level and high standard is important.
Particularly, in automobile parts, a part in which a sheet metal is processed as a material and functions as a rotating body, for example, a drum, a carrier, or the like configuring an automatic transmission is an important part which transmits engine output to an axle shaft. The part requires circularity or uniformity of a sheet thickness in a circumferential direction as a shape for decreasing friction or the like. In addition, since a forming type such as burring processing, drawing, ironing, or stretch forming is used when the part is formed, ultimate deformability which is represented by local elongation is significantly important.
Moreover, it is preferable to improve impact resistance, that is, toughness in the steel sheet used for the member, in which the impact resistance is a characteristic in which the member is not easily broken even though the member receives impact due to collision or the like after the formed member is mounted to an automobile as a part of the automobile. Particularly, when use of the member under a cold climate is considered, it is preferable to improve the toughness at low temperature (low-temperature toughness) in order to secure the impact resistance at low temperature. Thereby, it is important to increase the impact resistance of the steel. In addition, the impact resistance (toughness) is defined by vTrs (Charpy fracture appearance transition temperature) or the like.
That is, in a steel sheet for a part including the above-described part which requires uniformity of a sheet thickness, satisfying both of plastic isotropy and impact resistance (toughness) is required in addition to improved workability.
For example, in Patent Document 1, in order to satisfy both of high strength and various material characteristics which particularly contribute to formability, a manufacturing method of the steel sheet, which satisfies high strength, ductility, and hole expansibility by including a steel structure which has ferrite of 90% or more and the balance consisting of bainite, is disclosed.
However, in the steel sheet which is manufactured by applying the technique disclosed in Patent Document 1, the plastic isotropy is not disclosed at all. Thereby, for example, if it is assumed that the steel sheet of Patent Document 1 is applied to a part such as a gear which requires circularity or uniformity of the sheet thickness in the circumferential direction, unfair vibration due to eccentricity of the part or a decrease in the output due to friction loss is concerned.
Moreover, for example, in Patent Documents 2 and 3, a hot-rolled high tensile steel sheet, which has high strength and improved stretch flangeability by adding Mo and refining precipitates, is disclosed.
However, in the steel sheet to which the above-described technique disclosed in Patent Documents 2 and 3 is applied, since it is essential to add Mo, which is an expensive alloy element, by 0.07% or more, there is a problem that the manufacturing costs are increased. Moreover, in the technique disclosed in Patent Documents 2 and 3, the plastic isotropy is not disclosed at all. Thereby, if it is assumed that the steel sheet of Patent Documents 2 and 3 is applied to a part which requires circularity or uniformity of the sheet thickness in the circumferential direction, unfair vibration due to eccentricity of the part or a decrease in the output due to friction loss is concerned.
On the other hand, for example, in Patent Document 4, with respect to improvement in plastic isotropy of the steel sheet, that is, a decrease of the plastic anisotropy, a technique is disclosed which makes texture at austenite of a surface shear layer be adequate by combining endless rolling and lubricant rolling and decreases in-plane anisotropy of a r value (Lankford value).
However, the endless rolling is needed for preventing defective biting caused by slip between a roll caliber tool and a rolled material during rolling in order to perform the lubricant rolling having a small friction coefficient over the full length of a coil. Thereby, since equipment investment such as a rough bar joining device or a high-speed crop shear is accompanied to apply the technique of Patent Document 4, a burden is large.
In addition, for example, in Patent Document 5, a technique is disclosed which satisfies both of stretch flangeability and deep drawability by decreasing anisotropy of a r value in a steel sheet having strength level of 780 MPa or more which is obtained by compositely adding Zr, Ti, and Mo and ending finish rolling at high temperature of 950° C. or more.
However, since adding Mo, which is an expensive alloy element, of 0.1% or more is essential, there is a problem that the manufacturing costs are increased.
Research for improving toughness of a steel sheet has been advanced than conventional. However, a hot-rolled steel sheet for gas nitrocarburizing having high strength, improved plastic isotropy and toughness is not disclosed in the above-described Patent Documents 1 to 5.